The Gospel accounts were not written down until a generation after Christ. The modern Bible was not assembled until the year 331 at the earliest.
The books of Moses were inscribed from oral tradition around 600 BC, and compiled in to a book around 400 BC. (Moses himself lived around 1400 BC)
The printing press was not around until 1440 AD. Before then, very few owned Bibles. (Very few were literate as well...)
Oral tradition and rabbinical/church interpretation and tradition are older and more established methods of understanding the faith than the written bible.
Consider this. If the book itself were so important, why did Jesus preach and not write? Why didn't Jesus write his own gospel?